How to stress-test your Coast FIRE number (practical guide)

Hitting a Coast FIRE number on a smooth-return calculator feels decisive. It should not be. Coast math answers: “If returns equal my assumed real rate every year until traditional retirement age, is my pile large enough that contributions can drop?” Sequence risk, lower returns, and life plumbing can still make that “yes” fragile. This guide is a practical stress-test workflow you can run in an afternoon.

What you are testing (and what you are not)

• You are testing whether a coast decision looks robust under worse assumptions and path noise.
• You are not proving you can retire early, quit healthcare planning, or ignore taxes.
• Success rates from a toy Monte Carlo are model outputs under stated rules — not the probability your life works out.
• Coast ≠ Barista. If part-time income covers a spending gap, use Barista math separately.

Step 1 — Write the deterministic baseline

Open the Coast FIRE calculator. Set annual spending, withdrawal rate, current age, traditional retirement age, current portfolio, and real return. Write down: full FIRE = spending ÷ SWR; coast ≈ FIRE ÷ (1+r)^n with n = retirement age − current age; shortfall or surplus vs portfolio today. Export CSV or copy the share link so the baseline is recoverable.

Step 2 — Sensitivity before simulation

• Drop real return by ~1 percentage point (e.g. 5% → 4%). Recompute coast.
• Drop SWR (e.g. 4% → 3.5% or 3%). Full FIRE and coast both rise.
• Raise spending 10% (lifestyle creep or healthcare realism). Recompute.
• Use Scenario compare: pin baseline A, change one lever at a time as B.
• If surplus disappears under two mild stresses, treat “already coasting” as soft, not permanent.

Step 3 — Free sequence stress test (Coast tool)

On Coast FIRE, open the stress-test panel. It runs 1,000 paths with the same mean real return and a chosen volatility (12% / 15% / 18% presets), contributions = 0, target = full FIRE, seed 42 for reproducibility. Read: success rate (paths with terminal ≥ full FIRE), p10 / median / p90 terminals, sample paths, and the histogram. Prefer interpreting the band of outcomes over a single green success percentage.

Step 4 — How to read the results without false precision

• High success + p10 still near target → model says more cushion under this toy path model.
• OK median but weak p10 → order risk bites; keep savings, lower spending target, or delay the coast decision.
• Changing σ from 12% to 18% swings results a lot → your conclusion depends on volatility assumptions; say so out loud.
• Never treat 1,000 i.i.d. annual shocks as historical truth. See Monte Carlo vs historical cycles for the other family of tools.

Worked sketch (illustrative only)

Spending $60k, SWR 4% → FIRE $1.5M. Age 40 → 65 (25 years), r = 5% real → coast ≈ $443k. Portfolio $500k: constant-return model says past coast. Sensitivity at 4% real and 3.5% SWR can erase that surplus. A free stress test may still show a non-trivial share of paths finishing under $1.5M if σ is high and the horizon is long. Decision: maybe reduce contributions gradually, keep emergency funds, re-run yearly — not “markets are guaranteed.”

Step 5 — Life gates outside the formula

• Healthcare plan for remaining working years and any gap years.
• Emergency fund separate from the invested coast pile.
• High-interest debt and near-term cash needs named, not assumed away.
• Rule against lifestyle creep silently raising full FIRE after you “coast.”
• Partner / co-planner saw the same share link or CSV.

Go / no-go hygiene

• Go (cautious): surplus under base + mild stresses; stress-test tails acceptable to you; life gates green.
• Hold: base case coasts but stress cases do not — keep contributing a floor amount.
• No-go: base case only works at optimistic r and SWR — do not change savings on that alone.
• Use the free Coast assumptions checklist as a pre-flight.

FAQ

• Is a 90% success rate “safe”? It is 90% of simulated paths under this model definition — not a warranty.
• Should I coast if stress test is “ugly” but constant return is green? Prefer ranges; often keep a savings floor.
• Does stress test replace historical tools? No — it is complementary and simpler. Use both families when stakes are high.
• Taxes? Not in the model. Effective after-tax plans can need a larger pile.

Run the workflow on the Coast FIRE calculator, read Methodology for formulas, Approach for what stays free, and the sequence-risk guide for path concepts. Educational only — not financial, tax, or investment advice.